The present invention relates to a speed control system and method for universal (series-type, a-c or d-c) electric motors.
Because of inherent wide speed fluctuations with varying load, series motors have rarely been used in automatic control systems despite their very high starting torque and their ability to function throughout an extreme range of speed. Another disadvantage of series motors has been their tendency to run at dangerously high speeds at light loads. Accordingly, it is an object of this invention to provide an improved series motor control system which will effectively eliminate speed fluctuations and prevent overspeeding. Another object of the present invention is to provide a simple, continuous (i.e., stepless), inexpensive speed control system which will permit the precise setting and maintaining of a selected speed within a wide range regardless of the necessary torque.
Typical prior art series motor speed controls are either relatively complex and expensive or, while simple and inexpensive, are of limited capability as to speed range and degree of speed regulation with load change. The latter also, when operated with a load of the type having a high starting friction and inertia, often produce an initial sudden burst of speed, rising far above the desired value. It is accordingly a further object of this invention to provide a speed control system which has smooth starting and stopping characteristics in addition to providing full-rated torque output at slow speeds.
Another object of this invention is to provide an improved series motor speed control system which is reliable and rugged in operation, has a long operating life and is economical and convenient to operate.
Another of the primary objectives of the present invention is to extend the capabilities of universal motors with respect to starting torque and speed range, by means of a motor controller providing a high degree of regulation of speed over a wider range of loads and speed than possible with prior art devices. Other objectives are to provide a simple, inexpensive device, not requiring extra leads from the motor.
An additional object of the invention is to utilize full-wave d-c current to operate the motor. Full-wave d-c operation, compared with half-wave or a-c operation, results in improved smoothness of operation with reduced noise and vibration, higher torque capability, substantially improved efficiency, and extended motor life due to reduced peak currents and thus reduced brush and commutator erosion.
The prior art teaches numerous systems for controlling the speed of universal electric motors. For example, the most precise control systems use a separate tachometer for sensing speed, with the tachometer reading serving as input to a feedback circuit. Some prior art systems base the feedback on the motor armature voltage, which requires separate leads to be brought out from the motor to measure the voltage or requires that the motor be operated only on half wave. Other prior art systems base the feedback on the motor current, which is a useful measure of speed only for fixed loads. Neither armature voltage nor motor current is a unique function of speed independent of the applied load.
One prior art patent, U.S. Pat. No. 2,978,624 (Smith-Vaniz), teaches the use of the instantaneous ratio of motor terminal voltage to motor terminal current, i.e., the terminal impedance, in a series motor control system. The Smith-Vaniz patent uses a magnetic amplifier feedback system to sense the instantaneous values of the terminal voltage and the terminal current, which are inductively smoothed, and presents a negative output impedance to the motor of a magnitude to match the effective resistance presented by the motor at the desired speed.
The present invention provides a much more precise speed control system than the Smith-Vaniz system, as will be discussed below, and does not require the costly and bulky magnetic amplifier of Smith-Vaniz.
These, and other objects, aspects, and advantages of the present invention will be more fully understood from a consideration of the following specification and claims in conjunction with the accompanying diagrams.